[v2] i2c: mux: pca954x: allow management of device idle state via sysfs

Message ID 20190129172352.24466-1-robertshearman@gmail.com
State Superseded
Headers show
Series
  • [v2] i2c: mux: pca954x: allow management of device idle state via sysfs
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Commit Message

Robert Shearman Jan. 29, 2019, 5:23 p.m.
From: Robert Shearman <robert.shearman@att.com>

The behaviour, by default, to not deselect after each transfer is
unsafe when there is a device with an address that conflicts with
another device on another pca954x mux on the same parent bus, and it
may not be convenient to use the platform data or devicetree to set
the deselect mux, e.g. when running on x86_64 when ACPI is used to
discover most of the device hierarchy.

Therefore, provide the ability to set the idle state behaviour using a
new sysfs file, idle_state as a complement to the method of
instantiating the device via sysfs. The possible behaviours are
disconnect, i.e. to deselect all channels from the mux, as-is (the
default), i.e. leave the last channel selected, and set a
predetermined channel.

The current behaviour matches most closely with as-is, although the
driver platform data can be used to select a mix of disconnect and
as-is on a per-channel basis, and this capability is preserved.

Signed-off-by: Robert Shearman <robert.shearman@att.com>
---

Changes in v2:
 - change from exposing deselect mask to idle state with more options
   and less implementation specific
 - fix style issues
 - don't fail the create of the mux device if the device attribute
   file in sysfs failed to create
 
 .../ABI/testing/sysfs-bus-i2c-devices-pca954x | 20 ++++
 drivers/i2c/muxes/i2c-mux-pca954x.c           | 91 ++++++++++++++++++-
 2 files changed, 108 insertions(+), 3 deletions(-)
 create mode 100644 Documentation/ABI/testing/sysfs-bus-i2c-devices-pca954x

Patch

diff --git a/Documentation/ABI/testing/sysfs-bus-i2c-devices-pca954x b/Documentation/ABI/testing/sysfs-bus-i2c-devices-pca954x
new file mode 100644
index 000000000000..bb1c2e538c5f
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-bus-i2c-devices-pca954x
@@ -0,0 +1,20 @@ 
+What:		/sys/bus/i2c/.../idle_state
+Date:		January 2019
+KernelVersion:	5.0
+Contact:	Robert Shearman <robert.shearman@att.com>
+Description:
+		Value that can be written to control the behaviour of
+		the multiplexer on idle. Possible values:
+		-2 - disconnect on idle, i.e. deselect the last used
+		     channel, which is useful when there is a device
+		     with an address that conflicts with another
+		     device on another pca954x mux on the same parent
+		     bus.
+		-1 - leave the mux as-is, which is the most optimal
+		     setting in terms of I2C operations and is the
+		     default mode.
+		0..<nchans> - set the mux to a predetermined channel,
+		     which is useful if there is one channel that is
+		     used almost always, and you want to reduce the
+		     latency for normal operations after rare
+		     transactions on other channels
diff --git a/drivers/i2c/muxes/i2c-mux-pca954x.c b/drivers/i2c/muxes/i2c-mux-pca954x.c
index bfabf985e830..c032396abdcc 100644
--- a/drivers/i2c/muxes/i2c-mux-pca954x.c
+++ b/drivers/i2c/muxes/i2c-mux-pca954x.c
@@ -50,6 +50,7 @@ 
 #include <linux/pm.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
+#include <dt-bindings/mux/mux.h>
 
 #define PCA954X_MAX_NCHANS 8
 
@@ -86,6 +87,9 @@  struct pca954x {
 
 	u8 last_chan;		/* last register value */
 	u8 deselect;
+	/* MUX_IDLE_AS_IS, MUX_IDLE_DISCONNECT or >= for channel */
+	s8 idle_state;
+
 	struct i2c_client *client;
 
 	struct irq_domain *irq;
@@ -250,19 +254,81 @@  static int pca954x_select_chan(struct i2c_mux_core *muxc, u32 chan)
 	return ret;
 }
 
-static int pca954x_deselect_mux(struct i2c_mux_core *muxc, u32 chan)
+static int __pca954x_deselect_mux(struct i2c_mux_core *muxc)
 {
 	struct pca954x *data = i2c_mux_priv(muxc);
 	struct i2c_client *client = data->client;
 
-	if (!(data->deselect & (1 << chan)))
-		return 0;
+	if (data->idle_state >= 0)
+		/* Set the mux back to a predetermined channel */
+		return pca954x_select_chan(muxc, data->idle_state);
 
 	/* Deselect active channel */
 	data->last_chan = 0;
 	return pca954x_reg_write(muxc->parent, client, data->last_chan);
 }
 
+static int pca954x_deselect_mux(struct i2c_mux_core *muxc, u32 chan)
+{
+	struct pca954x *data = i2c_mux_priv(muxc);
+
+	if (!(data->deselect & (1 << chan)))
+		return 0;
+
+	return __pca954x_deselect_mux(muxc);
+}
+
+static ssize_t idle_state_show(struct device *dev,
+				    struct device_attribute *attr,
+				    char *buf)
+{
+	struct i2c_client *client = to_i2c_client(dev);
+	struct i2c_mux_core *muxc = i2c_get_clientdata(client);
+	struct pca954x *data = i2c_mux_priv(muxc);
+
+	return sprintf(buf, "%d\n", READ_ONCE(data->idle_state));
+}
+
+static ssize_t idle_state_store(struct device *dev,
+				struct device_attribute *attr,
+				const char *buf, size_t count)
+{
+	struct i2c_client *client = to_i2c_client(dev);
+	struct i2c_mux_core *muxc = i2c_get_clientdata(client);
+	struct pca954x *data = i2c_mux_priv(muxc);
+	int val;
+	int ret;
+
+	ret = kstrtoint(buf, 0, &val);
+	if (ret < 0)
+		return ret;
+
+	if (val != MUX_IDLE_AS_IS && val != MUX_IDLE_DISCONNECT &&
+	    (val < 0 || val >= data->chip->nchans))
+		return -EINVAL;
+
+	i2c_lock_bus(muxc->parent, I2C_LOCK_SEGMENT);
+
+	WRITE_ONCE(data->idle_state, val);
+	if (val == MUX_IDLE_AS_IS)
+		data->deselect = 0;
+	else {
+		data->deselect = GENMASK(data->chip->nchans - 1, 0);
+		/*
+		 * Set the mux into a state consistent with the new
+		 * idle_state.
+		 */
+		if (data->last_chan || val != MUX_IDLE_DISCONNECT)
+			ret = __pca954x_deselect_mux(muxc);
+	}
+
+	i2c_unlock_bus(muxc->parent, I2C_LOCK_SEGMENT);
+
+	return ret < 0 ? ret : count;
+}
+
+static DEVICE_ATTR_RW(idle_state);
+
 static irqreturn_t pca954x_irq_handler(int irq, void *dev_id)
 {
 	struct pca954x *data = dev_id;
@@ -329,8 +395,11 @@  static int pca954x_irq_setup(struct i2c_mux_core *muxc)
 static void pca954x_cleanup(struct i2c_mux_core *muxc)
 {
 	struct pca954x *data = i2c_mux_priv(muxc);
+	struct i2c_client *client = data->client;
 	int c, irq;
 
+	device_remove_file(&client->dev, &dev_attr_idle_state);
+
 	if (data->irq) {
 		for (c = 0; c < data->chip->nchans; c++) {
 			irq = irq_find_mapping(data->irq, c);
@@ -444,6 +513,16 @@  static int pca954x_probe(struct i2c_client *client,
 			goto fail_cleanup;
 	}
 
+	if (data->deselect == GENMASK(data->chip->nchans - 1, 0))
+		data->idle_state = MUX_IDLE_DISCONNECT;
+	else
+		/*
+		 * Some channels may be deselected, but this is safer
+		 * than claiming idle-disconnect behaviour when
+		 * inspected.
+		 */
+		data->idle_state = MUX_IDLE_AS_IS;
+
 	if (data->irq) {
 		ret = devm_request_threaded_irq(dev, data->client->irq,
 						NULL, pca954x_irq_handler,
@@ -453,6 +532,12 @@  static int pca954x_probe(struct i2c_client *client,
 			goto fail_cleanup;
 	}
 
+	/*
+	 * The attr probably isn't going to be needed in most cases,
+	 * so don't fail completely on error.
+	 */
+	device_create_file(dev, &dev_attr_idle_state);
+
 	dev_info(dev, "registered %d multiplexed busses for I2C %s %s\n",
 		 num, data->chip->muxtype == pca954x_ismux
 				? "mux" : "switch", client->name);